1. Field of the Invention
The present invention relates to an electro-photographic image forming apparatus in which a toner image is formed by an electro-photographic process on the surface of a photosensitive belt passing around a plurality of rollers, and the toner image is transferred onto the surface of an intermediate transfer drum.
2. Description of the Prior Art
An electro-photographic image forming apparatus such as an electro-photographic copying machine or an electro-photographic printer generally comprises a charging device which uniformly charges the surface of a photosensitive drum with the photosensitive drum as the center, an exposing device which forms an electrostatic latent image by exposing the electrically charged surface of the photosensitive drum in accordance with image data, a developing device which develops the electrostatic latent image and transforms it into a toner image, a transfer device which transfers the toner image onto a recording paper that is fed, and a thermal fixing device which permits the recording paper carrying the toner image to pass in order to fix the toner image on the recording paper.
In order to form a color picture using an image forming apparatus of this type, it has been attempted to complete a color toner image on the surface of an intermediate transfer drum by forming four toner images of different colors (yellow, magenta, cyan and black) on the surface of the photosensitive belt and transferring these four toner images on the surface of the intermediate transfer drum in a superposed manner.
In such an image forming apparatus, the photosensitive belt is an expendable part which requires maintenance or which needs to be replaced after a predetermined time period.
At the time of maintenance work or replacement, the photosensitive belt is detached from the apparatus and then a new one is attached to the apparatus. The photosensitive belt, however, must be fitted in the apparatus in a proper position with respect to the developing devices that act upon the photosensitive belt to develop the electrostatic latent image and with respect to the intermediate transfer drum that transfers the toner image formed through the development, requiring cumbersome adjustment work at the time of maintenance operation and replacement.
A color printer which forms a colored toner image by transferring toner images of the colors formed on the photosensitive belt onto the intermediate transfer dram in a superposed manner, is disclosed in, for example, Japanese Patent Laid-Open. No. 256976/1988 and a mechanism for detachably attaching the photosensitive belt unit is disclosed in, for example, Japanese Patent Laid-Open No. 113277/1985.
In apparatus which effect color printing, however, a plurality of developing devices facing the photosensitive belt are installed, and it is important to precisely position the photosensitive belt so as to face, with precise gaps, the developing rollers of the developing devices.
It is also important to keep constant the width of contact of the photosensitive belt with the intermediate transfer drum since it deeply affects the quality of the printed image.